This article applies to 123-reg servers using Plesk
This article will explain what the difference is between a shared IP address and an exclusive IP address.
Shared IP address
A shared IP address can be used to run an unlimited number of websites over a HTTP connection.
Exclusive IP address
An exclusive IP address can be assigned to a single website only and is generally used when there is a website that needs an SSL certificate installed so that it can run secure HTTPS connections.